Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing (CMP), etching, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Metrology processes are used at various steps during a semiconductor manufacturing process to monitor and control one or more semiconductor layer processes. For example, during lithography, maintaining sharp focus between the reticle and the resist is vital. Sub-resolution assist features (SRAF) are especially useful where lithographic targets are particularly sensitive to focus. Sub-resolution assist features (SRAF) are very small mask features designed to improve processes margins and enhance resolution for isolated features through additional constructive or destructive interference, but are not intended to be printed on the resulting wafer. Existing lithographic technology requires deconvolution of dose and focus using some form of CD metrology data and separation of data to determine focus response.
While sub-resolution assist features are useful for assisting focus during lithography, existing sub-resolution assist features do not offer a reliable mechanism for maintaining focus. Consequently, it would be advantageous if an apparatus existed that is suitable for monitoring focus of a mask in a lithographic process.